Conventionally, piezoelectric sensors have been often used for measuring a pressure or a force. The piezoelectric sensor has excellent properties, e.g., no displacement of the sensor itself, a high speed of response and a small size. However, the piezoelectric sensor can not be used when the temperature of a sensor device exceeds Curie temperature. Thus, for example, when quartz is used as the sensor device, it can not be used in a temperature zone exceeding 400° C., and pressure (combustion pressure) variation can not be directly measured in the temperature zone exceeding 1000° C. when a fuel-air mixture in a combustion chamber is exploded in an internal combustion.
Meanwhile, for the purpose of being used at high temperature, it is proposed to use rare earth gallate (La3Ga5SiO14) single crystal having no Curie point (e.g., see Patent Document 1). A melting point of the rare earth gallate single crystal is 1500° C., and thus, it is possible to use it as the piezoelectric sensor even at the temperature zone exceeding 1000° C. However, in the rare earth gallate single crystal, its resistivity largely depends on the temperature, its resistivity becomes small at the high temperature zone and sufficient voltage can not be kept at the high temperature zone. Thus, it is problematic in that the combustion pressure variation can not be precisely measured.
Patent Document 1: JP Hei-10-54773-A